I believe that back when IM messages tended to be shorter, or at least when Twitter popularized the 120 character (or is it 140 now) limit, it made sense that the ENTER key would double up as a SAVE/SEND action because people didn't write long paragraphs.
However, since most applications now have a chat function/feature and that online collaboration is the norm, there seems to be some inconsistency in how the ENTER key works in different types of applications.
Commonly we see a separate SEND key (for example on LinkedIn when you try to send someone a message) so that the user might expect the ENTER key to work as a new line. There are also editors with markup/down languages that removes the need for an ENTER key in textareas.
Something that is frustrating and creates a poor user experience is when someone hits the ENTER key expecting to start a new line and instead ends up sending an incomplete message.
Is there a default behaviour on web or desktop (or mobile) applications these days where you might expect the ENTER key to work as a SAVE/SEND, and if not then how is this design pattern normally solved so there are no confusions.